Powdery coating composition and process for preparing the same

ABSTRACT

This invention relates to a powdery coating composition having excellent anti-caking ability, which comprises polymer particles having an average particle size of 0.5 to 200 mu  composed of a polymer having a softening point not exceeding 90 DEG C. as a core component and a polymer having a softening point of at least 100 DEG C. as an outer component covering said core component, and to a process for the preparation of such powdery coating composition.

DESCRIPTION OF THE PRIOR ART

Powdery resinous compositions and colored powdery resinous compositionshave heretofore been prepared by methods comprising pulverizing resinsor pigment-dispersed resins, or by methods comprising spray drying resinsolutions or latices to thereby remove solvents or water.

For instance, the following methods have been known as methods forpreparing powdery resinous compositions.

1. A method comprising agitating a water-soluble, thermoplastic polymerunder heating in water containing a specific dispersing agent or wettingagent to thereby disperse the polymer in water, and removing water fromthe resulting dispersion of fine particles (see U.S. Pat. No. 3,101,329,U.S. Pat. No. 3,449,291 and U.S. Pat. No. 3,622,555).

2. A method comprising melt-blending polyesters having different glasstransition temperatures, and pulverizing the resulting blend (see U.S.Pat. No. 3,382,295).

3. A method comprising forming a powdery product composed of an acrylicpolymer as a core component and an outer covering component of anotheracrylic polymer having a higher glass transition temperature than thatof the core component polymer, and mixing the powdery product with ablocked isocyanate compound to obtain a thermoplastic molding powder.(see U.S. Pat. No. 3,659,003).

4. A method comprising adding a specific water-immiscible solvent to anaqueous dispersion of a polymer, and removing the solvent and watersimultaneously by distillation to obtain a powdery product (JapanesePatent Application Laid-Open Specification No. 45550/73).

5. A method comprising polymerizing acrylic monomers in the presence ofa stabilizer in a solvent capable of dissolving monomers but incapableof dissolving polymers formed from said monomer (Japanese PatentApplication Laid-Open Specification No. 17888/73).

Powdery compositions obtained according to these conventional methodsare defective in that they are not suitable as coating compositions orthey are very poor in storage stability, especially resistance tocaking.

When improvement of the caking resistance is desired, resins having asoftening point of at least 90°C., preferably at least 130°C., aregenerally employed. In the case of a powdery coating compositionincluding a resin having a softening point of 130°C. or higher, in orderto obtain a smooth coating surface, it is necessary to conduct thebaking at a high temperature exceeding 150°C. to thereby make the resinuniformly flowable. Especially in the case of a thermosetting resin, ifsuch flow temperature approximates the curing temperature of the resin,there is inevitably brought about a disadvantage that the resin is curedwhile flowing and hence, a smooth coating film cannot be obtained. Forthis reason, a great difference between the flow temperature and thecuring temperature is indispensable. In general, it is necessary thatthe curing temperature should be higher by at least 70°C. than thesoftening point (flow temperature).

As pointed out above, in conventional powdery coating compositions, forexample, thermoplastic resin compositions, in order to improve theanti-caking ability, limitations are imposed on the softening point ofthe resin. Further, in the case of thermosetting resin compositions, inorder to obtain an improved anti-caking property, limitations areimposed on such factors as the flow temperature, the melt viscosity andthe curing temperature. Accordingly, powdery coating compositionsavailable according to the conventional methods are limited to veryspecific ones.

SUMMARY OF THE INVENTION

This invention relates to a powdery coating composition having excellentanti-caking ability, which comprises polymer particles composed of apolymer having a softening point not exceeding 90°C. as a core componentand a polymer having a softening point of at least 100°C. as an outercomponent covering said core component, and to a process for thepreparation of such powdery coating composition.

DETAILED DESCRIPTION OF THE INVENTION

This invention has been completed as a result of our research work madewith a view to developing a powdery coating composition free of theabove-mentioned defects and disadvantages involved in conventionalpowdery coating compositions, and the objects of this invention are asfollows.

1. To provide a powdery coating composition having an improvedanti-caking ability (by the term "anti-caking ability" is meant aresistance to an undesired phenomenon called caking, namely a phenomenonthat when a slight pressure is applied to a powdery composition instorage or transportation, the resin particles adhere to one another toform agglomerates).

2. To provide a powdery coating composition which exhibits improvedmelting point and flow characteristics at the baking step.

3. To provide a powdery coating composition which can give a film havingexcellent surface smoothness and flexibility even by low temperaturebaking.

4. To provide a thermosetting powdery resin composition having improvedflow characteristics at the film-forming step.

In short, the main object of this invention is to provide a powderycoating composition which is excellent in storage stability andanti-caking ability though it has a low softening point and a low meltviscosity.

The "softening point" referred to in the instant specification andclaims is one determined by the ring-ball method specified in JapaneseIndustrial Standard, JIS-K-2531.

As the process for preparing the powdery coating composition of thisinvention, there can be mentioned, for example, the following twoprocesses.

I. a process comprising polymerizing in water a monomer giving a polymerhaving a softening point not exceeding 90°C. to obtain an aqueousdispersion of a polymer having a softening point not exceeding 90°C.,adding a monomer giving a polymer having a softening point of at least100°C. to the aqueous dispersion, polymerizing said monomer to obtain anaqueous dispersion of fine particles composed of a core component and anouter covering component, separating the fine polymer particles from theaqueous phase and drying them.

Ii. a process comprising conducting the steps of (i) dispersing orsuspending a polymer (A) having a softening point not exceeding 90°C. asa core component in a solvent selected from water and organic solvents,and (ii) dissolving a polymer (B) having a softening point of at least100°C. as an outer covering component in said solvent, said solventbeing incapable of dissolving said polymer (A) but capable of dissolvingsaid polymer (B), said steps (i) and (ii) being conducted in an optionalorder or simultaneously, and then spray drying the resultingcomposition.

Since fine particles of this invention obtained according to theforegoing processes have their surface coated with a resin having asoftening point of at least 100°C., their surface characteristics arenot substantially different from those of conventional anti-cakingpowder particles. Further, since the fine particles of this inventioncontain in the interior thereof a resin having a lower softening point,the melting temperature and flow characteristics of the particles at thebaking step are highly improved. Therefore, films having excellentsurface smoothness can be prepared from the fine particles of thisinvention, even if the baking is conducted at a low temperature.

According to the above process (I), a powdery composition of thisinvention is prepared by employing monomers as starting materials, andaccording to the above process (II), a powdery composition of thisinvention is prepared by employing polymers as starting materials.

As the monomer giving a softening point not exceeding 90°C., which isused in the process (I), there can be used one monomer or a mixture oftwo monomers or more. In this invention, it is desired that at least apart, preferably 5 to 60% by weight, of the starting monomer componentgiving a polymer having a softening point not exceeding 90°C. is amonomer, a homopolymer of which has a glass transition temperature(hereinafter abbreviated to "Tg") lower than 0°C.

As the monomer giving a homopolymer having a Tg lower than 0°C., therecan be mentioned, for example, ethyl acrylate, butyl acrylate,2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl methacrylate,n-decyl methacrylate, tetradecyl methacrylate, vinyl propionate, dibutylitaconate, dibutyl fumarate and the like.

As the monomer giving a homopolymer having a Tg higher than 0°C., therecan be mentioned, for example α-methylstyrene, styrene, vinyltoluene,methyl acrylate, n-tetradecyl acrylate, methyl methacrylate, ethylmethacrylate, n-propyl methacrylate, butyl methacrylate, isobutylmethacrylate, tert-butyl methacrylate, cyclohexyl methacrylate,acrylonitrile, vinyl acetate, dimethyl itaconate, acrylic acid,methacrylic acid, itaconic acid, crotonic acid, maleic anhydride,acrylamide, N-methylol acrylamide, N-n-butoxymethyl acrylamide,diacetone acrylamide, trimethylamine methacrylimide,1,1-dimethyl-1-(2-hydroxypropyl)-amine methacrylimide,1,1-dimethyl-1-(2,3-hydroxypropyl)-amine methacrylimide, β-hydroxyethylmethacrylate, β-hydroxypropyl methacrylate, glycidyl methacrylate,dimethylaminoethyl methacrylate, tetrahydrofuryl methacrylate and thelike. These monomers are used singly or in the form of a mixture of twoor more of them.

In this invention, the polymer having a softening point of at least100°C. is prepared from a monomer exemplified above, and a suitablecombination of two or more of such monomers can also be used. In thelatter case, it is desired that at least 60% by weight of the startingmonomer mixture is constituted by a monomer, a homopolymer of which hasa Tg higher than 0°C., especially a Tg higher than 50°C.

Glass transition points of homopolymers of typical monomers to be usedin the process (I) are illustrated in Tables 1 and 2 given below.

                  Table 1                                                         ______________________________________                                        Monomers Giving Homopolymers Having a Tg                                      Lower than 0°C.                                                        Monomer         Tg (°C.) of Homopolymer                                ______________________________________                                        ethyl acrylate  -22                                                           isopropyl acrylate                                                                            - 5                                                           n-butyl acrylate                                                                              -54                                                           2-ethylhexyl acrylate                                                                         -85                                                           cyclohexyl acrylate                                                                           -15                                                           n-hexyl methacrylate                                                                          - 5                                                           n-octyl methacrylate                                                                          -20                                                           n-decyl methacrylate                                                                          -70                                                           lauryl methacrylate                                                                           -65                                                           tetradecyl methacrylate                                                                       - 9                                                           2-ethylhexyl methacrylate                                                                     -20                                                           vinyl propionate                                                                              - 2                                                           ______________________________________                                    

                  Table 2                                                         ______________________________________                                        Monomers Giving Homopolymers Having a Tg                                      Higher than 0°C.                                                       Monomer          Tg (°C.) of Homopolymer                               ______________________________________                                        methyl acrylate   8                                                           tert-butyl acrylate                                                                            41                                                           n-tetradecyl acrylate                                                                          20                                                           methyl methacrylate                                                                            105                                                          ethyl methacrylate                                                                             65                                                           n-propyl methacrylate                                                                          33                                                           isopropyl methacrylate                                                                         81                                                           n-butyl methacrylate                                                                           20                                                           iso-butyl methacrylate                                                                         48                                                           tert-butyl methacrylate                                                                        107                                                          sec-butyl methacrylate                                                                         60                                                           phenyl methacrylate                                                                            110                                                          cyclohexyl methacrylate                                                                        66                                                           2-hydroxyethyl methacrylate                                                                    55                                                           hydroxypropyl methacrylate                                                                     26                                                           acrylic acid     106                                                          methacrylic acid 185                                                          vinyl acetate    29                                                           acrylonitrile    96                                                           styrene          100                                                          ______________________________________                                    

Glass transition temperatures Tg and softening points of examples ofpolymers having a softening point not exceeding 90°C. and polymershaving a softening point of at least 100°C., which are prepared from theforegoing monomers, are shown in Tables 3 and 4 given below.

                                      Table 3                                     __________________________________________________________________________    Polymers Having a Softening Point Not Exceeding 90°C.                  Monomer Giving Homopolymer Having Tg Lower than 0°C.                                               Monomer Giving Homopolymer Having                                                                     Copolymer PQ              (% by weight) (Monomer P)   Higher Than 0°C. (% by weight)(Monomer                                 Q)                                                __________________________________________________________________________    2-ethylhexyl                                                                          ethyl acrylate                                                                         n-hexyl methacrylate                                                                     methyl acrylate                                                                        ethyl meth-                                                                          styrene      Softening            acrylate                             acrylate       Tg   Point                (Tg=-85°C.)                                                                    (Tg=-20°C.)                                                                     (Tg=-5°C.)                                                                        (Tg=8°C.)                                                                       (Tg=65°C.)                                                                    (Tg=100°C.)                                                                    (°C.)                                                                       (°C.)         __________________________________________________________________________                                100                      8   10-50                10                                   90             40   50-90                20                                          80      40   50-90                20                                   40     40      30   30-80                20                                   80             17   20-70                20                          20              60      25   30-80                        20                  40              40      30   35-85                        10                  70              20      20   30-80                        40                                  60      38   40-90                        40                           20     40      33   35-90                        40                  20       20     20      20   35-80                        50                                  50      30   35-80                                 10         70              20      30   35-80                                 10         40       20     30      40   30-90                                 10         40       40     10      35   40-80                __________________________________________________________________________

                                      Table 4                                     __________________________________________________________________________    Polymers Having a Softening Point of At Least 100°C.                   Monomer Giving Homopolymer of Low Tg                                                              Monomer Giving Homopolymer of High Tg                                                                      Copolymer ST                 (% by weight) (Monomer S)                                                                         (% by weight) (Monomer T)                                 __________________________________________________________________________    ethyl acrylate                                                                         n-butyl methacrylate                                                                     styrene                                                                              2-hydroxyethyl                                                                        methacrylic acid                                                                       Tg   Softening Point                                         methacrylate     (°C.)                                                                       (°C.)                 (Tg=-20°C.)                                                                     (Tg=20°C.)                                                                        (Tg=100°C.)                                                                   (Tg=55°C.)                                                                     (Tg=185°C.)                         __________________________________________________________________________    10                  90                      87   100 - 120                    5                   75     10      10       112  130 - 150                    5                   80     10      5        85   100 - 120                    5        10         80             5        83   100 - 115                    20                  80                      68   higher than 100                       20         70     10               64   higher than 105                                  70     25      5        81   105 - 125                                        90             10       112  120 - 150                             10         80     10               80   100 - 120                             10         75     10      5        87   103 - 125                                        100                     100  120 - 140                    __________________________________________________________________________

In view of the flow characteristics of the powdery resin composition atthe baking step, it is generally preferred that such homopolymers orcopolymers have a number average molecular weight of about 1,500 toabout 30,000.

In the process (I) of this invention, by employing a suitablecombination of a monomer giving a polymer of high Tg and a monomergiving a polymer of low Tg, it is possible to obtain a stable powderycomposition comprising a core composed of a low-softening-point polymerhaving a softening point not exceeding 90°C. and an outer cover composedof a high-softening-point polymer having a softening point of at least100°C. More specifically, one or more monomers selected so as to give apolymer having a softening point not exceeding 90°C. are dispersed inwater to form an aqueous dispersion and suspension polymerization oremulsion polymerization is carried out in the presence of apolymerization initiator to obtain an aqueous polymer dispersion. Ingeneral, the polymerization is conducted at a temperature ranging fromroom temperature to about 120°C. for 30 minutes to 8 hours. Then, one ormore monomers selected so as to give a polymer having a softening pointof at least 100°C. are added together with a polymerization initiator tothe above polymer dispersion, and the polymerization is carried outunder conditions similar to those mentioned above.

Selection of monomers for obtaining a polymer having a softening pointnot exceeding 90°C. or a polymer having a softening point of at least100°C. can be performed by prescribing the Tg which is one of thefactors controlling the softening point and determining the amounts andkinds of monomers by conducting the calculation based on such prescribedTg and Tg's of homopolymers of the monomers.

Further, tables such as Tables 3 and 4 are prepared in advance, andbased on the data of such tables, the kinds and amounts of monomerswhich will give a polymer having a desired softening point are selected.

In the process (I), in order for the outer polymer to cover sufficientlythe core polymer, it is desired that both the polymers beelectrostatically attracted to each other, for instance, by making acationic active agent present in one of the two polymers and making ananionic active agent present in the other polymer.

Dispersing conditions such as the agitation rate are preferably soselected that the average size of the final polymer particles is withinthe range of 0.01 to 1000μ, especially 0.5 to 200μ.

In the above-mentioned two kinds of polymerization, as thepolymerization initiator, there can be used, for instance, inorganicperoxides such as ammonium persulfate, potassium persulfate, hydrogenperoxide and percarbonates, organic peroxides such as acyl peroxides(e.g., benzoyl peroxide), alkyl hydroperoxides (e.g., tert-butylhydroperoxide and p-menthane hydroperoxide) and dialkyl peroxides (e.g.,di-tert-butyl peroxide), and nitrile compounds such as α,α'-azo-bisisobutyronitrile. Further, combinations of inorganic or organicperoxides and reducing agents can be used as redox catalysts. As thereducing agent, there can be mentioned, for instance, potassium acidsulfite, potassium sulfite, sodium acid sulfite, sodium sulfite, sodiumhydrosulfite, potassium hydrosulfite, sodium formaldehyde sulfoxylate,potassium formaldehyde sulfoxylate and zinc formaldehyde sulfoxylate.The amount of the catalyst is not particularly critical in thisinvention, but in general, the catalyst is used in an amount of 0.01 to5% by weight of the total monomers. If required, a known chain transferagent can be used in order to adjust the molecular weight.

In addition, if required, known anionic, cationic or non-ionicsurfactants, suspension stabilizers, emulsifiers and other additives maybe employed to disperse the starting monomers or particles of theresulting polymer stably in water. Moreover, a water-solublemacromolecular substance usually employed in this field can be used as aprotective colloid.

Ratios of the monomers used in the above-mentioned two-stagedpolymerization are so selected that in the final polymer particles, theweight ratio of the non-volatile component of the inner core polymer ofa low softening point to the non-volatile component of the outercovering polymer of a high softening point is within a range of from 3/1to 2/3, preferably from 2/1 to 1/1. When the amount of the outercovering polymer is too large, the softening point and flowcharacteristics of the final polymer particles are substantiallydominated by those of the outer polymer. When the amount of the corepolymer is too large, the intended improvement of the anti-cakingability cannot be attained because of the influence of the low softeningpoint of the core polymer present in too large an amount. Therefore, itis preferred that the ratio of the amounts of the core polymer and outercovering polymer be adjusted within the above range.

The intended powdery coating composition can be obtained by separatingfine polymer particles from the thus obtained aqueous polymer dispersionor emulsion according to a known method and drying the separated polymerparticles. The above separation can be accomplished according to knowntechniques such as filtration. Drying of the separated polymer particlescan also be accomplished by known techniques such as spray drying,freeze drying and vacuum drying. In the case of spray drying, it isdesired that the inlet temperature of the drying chamber be maintainedabove 100°C. and that the outlet temperature be maintained below 100°C.

In the above process (I), if a pigment is incorporated in the polymerparticles, there can be adopted a method comprising dispersing a pigmentin the monomer and then polymerizing the monomer dispersed in water, amethod comprising dispersing a pigment in water, then dispersing themonomer into the aqueous dispersion of the pigment and then polymerizingthe monomer, and other similar methods. In general, customary organicand inorganic pigments are used. They may be employed singly or in theform of admixtures of two or more of them. If necessary, apigment-dispersing agent is employed in combination with the pigment.

In the fine particles obtained according to the above-mentioned method(I), both the outer polymer and core polymer are bonded to each othermerely physically, or they are partially reacted with each other. Eitherof these two bonding states can be optionally attained by controllingthe reaction conditions such as the temperature, time and catalystamount appropriately. In each of these bonding states, the intendedobjects of this invention can be fully attained.

The process (II) will now be described.

The step (i) of dispersing or suspending a polymer (A) in water or anorganic solvent incapable of dissolving the polymer (A) therein can beaccomplished by the following methods.

a. A method comprising dispersing the powdery polymer (A) in water or anorganic solvent (hereinafter referred to merely as "solvent"). Forinstance, there can be mentioned a method in which the powdery polymer(A) is mechanically agitated in the solvent to thereby disperse it inthe solvent, or a method in which the powdery polymer (A) is dispersedor suspended in the solvent with the aid of a dispersing agent. (Asmeans for obtaining the starting powder of the polymer (A), there can beadopted known methods such as mechanical pulverization methods, e.g.,the air jet pulverization method and the ball mill pulverization method,and liquid resin or latex spray drying methods.)

b. A method comprising emulsifying and dispersing the polymer (A)directly in the solvent. This method is employed when the polymer (A) isliquid.

c. A method in which a commercially available latex, resin emulsion orsuspension-polymerized resin is employed as it is.

d. A method comprising heating the polymer (A) in the solvent at atemperature higher than the melting point of the polymer (A), forciblyemulsifying the polymer (A) and cooling it.

e. A method comprising mixing a small amount of the polymer (A) with asolvent capable of dissolving the polymer (A), dispersing the mixtureunder agitation in a large amount of said solvent incapable ofdissolving the polymer (A), and then removing the abovepolymer-dissolving solvent from the resulting dispersion.

f. A method comprising polymerizing the polymer (A)-constituting monomerin the solvent as in the above-mentioned process (I) to thereby obtain adispersion of the polymer (A).

The step (ii) of the process (II) can be accomplished only by dissolvingthe polymer (B) in the solvent. In case the solubility of the polymer(B) is low, it is possible to heat the system to dissolve the polymer(B), as long as the polymer (A) is not dissolved by such heating.

Either step (i) or step (ii) can be conducted first and it also ispossible to conduct them simultaneously.

Thus, according to this invention, there can be obtained a suspension offine polymer particles composed substantially of two layers, even if theresulting dispersion is not particularly allowed to stand still for acertain period of time.

As the polymer (A) of a softening point not exceeding 90°C., there canbe mentioned, for instance, alkyd resins, modified alkyd resins (alkydresins modified with rosin, phenol, styrene, an acrylic compound or thelike), phenol resins, natural resin-modified phenol resins, maleicresins, fumaric resins, natural resin-modified maleic resins, naturalresin-modified pentaerythritol resins, cellulose derivatives, estergums, rosin, petroleum resins, cumarone resins, indene resins, polyesterresins, polyamide resins, epoxy resins, polycarbonate resins, styreneresins, butyral resins, vinyl resins, acrylic resins, chlorinatedrubbers, ketone resins, xylene resins, melamine resins, benzoguanamineresins, urea resins, blocked isocyanate resins, polyurethane resins andthe like. These resins can be used in the form of admixtures of two ormore of them.

Water and various commercially available organic solvents can be used asthe solvent incapable of dissolving the polymer (A) but capable ofdissolving the monomer (B). For instance, ordinary organic solvents ofalcohol, ether, ester and ether alcohol types are employed. Selection ofa suitable solvent is performed mainly in view of the kinds of polymers(A) and (B) actually employed. In order to attain a high spray-dryingefficiency, it is desired that a solvent having a boiling point notexceeding 150°C. be employed.

As the polymer (B) which is water-soluble and has a softening point ofat least 100°C., there can be mentioned, for instance, polyester resins,cellulose derivatives, acrylic resins, alkyd resins and the like. It ispossible to employ two or more of these resins in combination.

As the polymer (B) which is soluble in an organic solvent and has asoftening point of at least 100°C., there can be mentioned cellulosederivatives, acrylic resins and other commercially available resins.They are used singly or in the form of admixtures of two or more ofthem.

In each of the foregoing resins to be used as the polymer (B) in thisinvention, the softening point is controlled to a level of at least100°C. For instance, in the case of polyester resins, the softeningpoint can be controlled by the combination of a polybasic acid and apolyhydric alcohol. In the case of acrylic resins (copolymer resins),the softening point can be controlled by determining the glasstransition temperature of a homopolymer of a monomer to be used andselecting a suitable comonomer based on the thus determined glasstransition temperature.

In the process (II) of this invention, ordinary organic or inorganicpigments, dyes, fillers and other additives may be incorporated ineither or both of the polymers (A) and (B). In case the polymer (A) or(B) is a solid resin, incorporation of a pigment can be accomplished bymeans of a heating roll, a heating kneader, an extruder or the like. Incase the polymer (A) or (B) is a liquid resin or latex, a three-rolldisperser, a ball mill, a sand grinder and other ordinary dispersers canbe used for incorporation of a pigment into the polymer. In casedifferent pigments are incorporated into the polymers (A) and (B),respectively, a multi-color film can be obtained from the resultingpolymer particles.

The intended polymer particles of this invention can be obtained byspray drying the liquid composition obtained by conducting the steps (i)and (ii) in an optional order or simultaneously.

The spray drying is performed by any of known conventional methods. Inorder to prevent agglomeration by cohesion of the resulting polymerparticles and obtain a good dry condition, it is desired that the spraydrying be conducted so that the air temperature at the inlet of thespray drier is higher than 100°C. and the air temperature at the outletof the spray drier is lower than 100°C. The spray drying is conductedunder such conditions that the size of the resulting polymer particlesis 0.01 to 1000μ, preferably 0.5 to 200μ.

The weight ratio of the non-volatile components of the polymer (A) andthe polymer (B) is within the range of from 40/60 to 100/0.1, preferablyfrom 50/50 to 100/1.

It is desired that the concentration of the liquid composition obtainedby dispersing the polymer (A) and dissolving the polymer (B) in thesolvent be within the range of 10 to 70% by weight, especially 30 to 60%by weight.

The powdery coating composition of this invention prepared according tothe above-mentioned process (I) or (II) is composed of a core polymerparticle having a low softening point (not exceeding 90°C.) and an outercovering polymer having a high softening point (at least 100°C.). Bydint of this specific particle structure, the powdery coatingcomposition of this invention has very excellent anti-caking ability,though it includes as one component a polymer having a low softeningpoint, which readily undergoes caking. Further, by dint of the presenceof such low-softening-point polymer at the baking step the powderycoating composition of this invention melts at a much lower temperaturethan in the case of polymer particles composed solely of ahigh-softening-point polymer, and therefore, the powdery coatingcomposition of this invention exhibits excellent flow characteristics atthe baking step, with the result that a coating film having excellentgloss, appearance and surface smoothness and having a high flexibilitycan be obtained.

The powdery coating composition of this invention prepared according tothe above-mentioned process (I) or (II) is coated on a substrate to becoated, such as metal substrates, e.g., iron, aluminum, zinc orzinc-plated iron plates, plywoods, wooden boards, slate plates, plasticsand the like according to customary coating methods, such as a fluidizedbed coating method, a sprinkling method, an electrostatic powder coatingmethod, an electrostatic fluidized bed coating method, a spraying methodand the like. Then, the coated composition is heat treated at 100° to300°C. for 5 to 60 minutes. Thus is obtained a coating film having suchexcellent properties as mentioned above. In view of the flowcharacteristics at the film-forming step and the surface smoothness ofthe resulting film, it is desired that at the film-forming step (bakingstep) the powdery coating composition be heated and molten at atemperature higher by at least 10°C. than the softening point of theouter covering polymer.

This invention will now be illustrated more concretely by reference toExamples, in which all "parts" and "%" are on the weight basis unlessotherwise indicated.

EXAMPLE 1

A reaction vessel equipped with a stirrer, a thermometer and a nitrogengas generator was charged with 1,000 parts of water and 30 parts ofNikkol DDP-10 (phosphoric acid type surfactant manufactured by NipponSurfactant Industry Co., Ltd.), and the mixture was agitated to form anaqueous dispersion. In a nitrogen atmosphere, the temperature of thedispersion was elevated to 75°-80°C. Then, a mixture of monomers havingthe following formulation (1) was suspended in the aqueous dispersion.

    ______________________________________                                        Formulation (1):                                                              2-ethylhexyl methacrylate                                                                            260 parts                                              methyl methacrylate    170 parts                                              styrene                200 parts                                              2-hydroxyethyl methacrylate                                                                           50 parts                                              methacrylic acid        20 parts                                              dodecyl mercaptan       1 part                                                benzoyl peroxide        10 parts                                              ______________________________________                                    

The mixture was reacted for 1 hour at the above temperature, and amixture of monomers of the following formulation (2) was added dropwiseto the reaction mixture over a period of 1 hour.

    ______________________________________                                        Formulation (2):                                                              2-ethylhexyl methacrylate                                                                             20 parts                                              N-methylol acrylamide   40 parts                                              styrene                 100 parts                                             methyl methacrylate     100 parts                                             butyl methacrylate      35 parts                                              methacrylic acid        5 parts                                               Emcol-CC-36 (cationic surfactant                                                                      0.5 part                                              manufactured by Witco Chemical                                                Co., Ltd.)                                                                    ______________________________________                                    

After completion of the dropwise addition, the temperature was elevatedto 80°-85°C. and at this temperature polymerization was continued for 2hours to obtain a suspension of resin particles composed of a corepolymer of a low softening point having a Tg of 35°C. (calculated value:each of the Tg values given hereinafter is a calculated value) and asoftening point of 75°C. and a high-softening-point polymer coveringsaid core polymer, said outer covering polymer having a Tg of 75°C. anda softening point of 120°C. The thus obtained suspension was dried in aspray drier in which the inlet temperature was 200°C. and the outlettemperature was 100°C. Thus was obtained a powdery resin having anaverage particle size of 20μ. The number average molecular weight of oneresin particle was about 2,500.

When the resulting powdery resin was stored at 40°C. for 1 week toexamine the stability, it was found that no caking occurred and thepowder exhibited good stability. When this powder was coated on an ironplate and baked at 140°C. for 30 minutes, a smooth and transparent filmwas obtained.

EXAMPLE 2

A reaction vessel equipped with a stirrer, a thermometer and a nitrogengas generator was charged with 500 parts of water, and 20 parts of PeloxOTP (anionic surfactant manufactured by Kao Soap Co., Ltd.), 10 parts ofammonium persulfate and 10 parts of potassium sulfite were dissolved inwater. Under agitation a mixture of monomers of the followingformulation (1) was added to the solution.

    ______________________________________                                        Formulation (1):                                                              ethyl acrylate          60 parts                                              lauryl methacrylate     50 parts                                              β-hydroxyethyl methacrylate                                                                      30 parts                                              styrene                 20 parts                                              butyl methacrylate      40 parts                                              titanium dioxide (white pigment)                                                                      100 parts                                             dodecyl mercaptan       2 parts                                               ______________________________________                                    

The inside atmosphere of the reaction vessel was replaced by nitrogengas, and the above mixture was reacted at 80°C. for 1 hour. Then, 5parts of ammonium persulfate and 5 parts of potassium sulfite were addedto the reaction mixture, and a mixture of monomers of the followingformulation (2) was added dropwise to the reaction mixture over a periodof 1 hour.

    ______________________________________                                        Formulation (2):                                                              methyl methacrylate  100 parts                                                styrene               70 parts                                                trimethylamine methaacrylamide                                                                      30 parts                                                ______________________________________                                    

After completion of the dropwise addition, the reaction was furtherconducted for 2 hours to form an aqueous suspension of a thermosettingresin composition composed of a white pigment-containing core polymer ofa low softening point having a Tg of -5°C. and a softening point of50°C. and an outer polymer of a high softening point covering saidlow-softening-point polymer. The suspension was dried in a spray drierin which the inlet temperature was 180°C. and the outlet temperature was90°C. to obtain a white powdery thermosetting resin composition havingan average particle size of 30μ. The number average molecular weight ofone polymer particle was about 4,000.

When this powder was stored at 40°C. for one week to examine thestability, it was found that no change occurred. When the powder wascoated on an iron plate and baked at 160°C. for 30 minutes, a smooth,thermoset film having solvent resistance was obtained.

EXAMPLE 3

A reaction vessel equipped with a stirrer, a thermometer and a nitrogengas generator was charged with 500 parts of water, and 20 parts of EmulNC (anionic surfactant manufactured by Kao Soap Co., Ltd.) and 30 partsof Emulgen 903 (HLB 8; non-ionic surfactant manufactured by Kao SoapCo., Ltd.) were dissolved in water. The temperature was elevated to80°C., and under agitation a pigment-dispersed mixture of monomershaving the following formulation (1) was suspended in the solution.

    ______________________________________                                        Formulation (1):                                                              lauryl methacrylate      20 parts                                             n-butyl methacrylate     150 parts                                            styrene                  30 parts                                             Phthalocyanine Blue (blue pigment)                                                                     50 parts                                             precipitated barium sulfate                                                                            150 parts                                             (extender pigment)                                                           azobisisobutyronitrile   5 parts                                              ______________________________________                                    

The reaction was carried out for 2 to 3 hours, and after completion ofthe reaction, a liquid mixture of the following formulation (2) wasadded dropwise to the reaction mixture over a period of 2 hours.

    ______________________________________                                        Formulation (2):                                                              methyl methacrylate     100 parts                                             styrene                 100 parts                                             butyl acrylate          100 parts                                             azobisisobutyronitrile  2 parts                                               Emcol CC-36 (same as used in                                                                          0.5 part                                                Example 1)                                                                  ______________________________________                                    

After completion of the dropwise addition, the temperature was elevatedto 85°C. and polymerization was continued for 6 hours at thistemperature, to obtain a suspension of resin particles composed of alow-softening-point core polymer having a Tg of 10°C. and a softeningpoint of 60°C. and a high-softening-point polymer covering saidlow-softening-point core polymer, said outer covering polymer having aTg of 60°C. and a softening point of 100°C. This suspension was filteredand the separated solid was vacuum dried to obtain a fine powder of anaverage particle size of 50μ having flowability. Either the core polymeror the outer covering polymer had a number average molecular weight ofabout 6,000.

The thus obtained powdery composition had excellent storage stability.When this powdery composition was coated on a substrate and baked at140°C. for 30 minutes, a smooth blue film having a good gloss wasobtained.

EXAMPLE 4

One hundred parts of titanium dioxide was dispersed uniformly in aliquid mixture of 150 parts of methyl methacrylate and 150 parts oflauryl methacrylate, and 3 parts of benzoyl peroxide was added to theresulting dispersion. The mixture was added to an aqueous solution of 15parts of Pelex OTP (anionic surfactant manufactured by Kao Soap Co.,Ltd.) and 15 parts of polyvinyl alcohol dissolved in 400 parts of water.The resulting suspension was charged in a reaction vessel equipped witha stirrer, a thermometer and a nitrogen gas generator, and thetemperature was elevated to 75°C. At this temperature, the reaction wascarried out for 2 to 3 hours.

Separately, a mixture of 100 parts of methyl methacrylate, 100 parts ofstyrene, 1 part of Emcol CC-36 (same surfactant as used in Example 1), 2parts of benzoyl peroxide and 1 part of dodecyl mercaptan was emulsifiedin an aqueous solution of 10 parts of polyvinyl alcohol in 200 parts ofwater. The resulting emulsion was added to the above reaction mixture inthe reaction vessel over a period of 30 minutes. After completion of thedropwise addition, the reaction was carried out at 85°C. for 40 minutesto obtain a suspension of resin particles composed of a whitepigment-containing core polymer having a Tg of 10°C. and a softeningpoint of 30°C. and an outer covering polymer having a softening point of120°C. and a Tg of 100°C., said outer polymer covering said corepolymer. The resin particles were separated by a centrifugal separatorand freeze dried to obtain white, flowable resin particles having anaverage particle size of 20μ. When the thus obtained powdery compositionwas subjected to the stability test for 1 week at 35°C., no change wasobserved. When this composition was coated on an iron plate and baked at140°C. for 30 minutes, a white film having excellent smoothness, glossand flexibility was obtained.

COMPARATIVE EXAMPLES 1 to 3

1. A powdery composition was prepared by mixing all the components ofthe formulation (1) and (2) used in Example 1 at one time polymerizingthe resulting mixture under the same conditions as described above, andconducting the spray drying.

2. A powdery composition was prepared by polymerizing the formulation(1) used in Example 1 and spray drying the resulting polymercomposition.

3. A powdery composition was prepared by polymerizing the formulation(2) used in Example 1 and spray drying the resulting polymercomposition.

With respect to each of the thus obtained powdery compositions, thespray drying property, i.e., the caking or cohesion tendency of thepowder at the spray drying step, the storage stability, i.e., theanti-caking ability in storage, and the film-forming property, i.e., thesmoothness of the resulting film, were tested to obtain results shown inTable 5.

                  Table 5                                                         ______________________________________                                        Powdery  Spray Drying                                                                              Storage     Film-Forming                                 Composition                                                                            Property.sup.1)                                                                           Stability.sup.2)                                                                          Property.sup.3)                              ______________________________________                                        Example 1                                                                              O           O           O                                            Comparative                                                                            Δ     X           O                                            Example 1                                                                     Comparative                                                                            X           X           O                                            Example 2                                                                     Comparative                                                                            O           O           X                                            Example 3                                                                     O: good, Δ: slightly bad, X: bad                                        ______________________________________                                         Notes:                                                                        .sup.1) inlet temperature of 200°C. and outlet temperature of          100°C.                                                                 .sup.2) stored at 40°C. for one week                                   .sup.3) baked at 140°C. for 30 minutes                            

EXAMPLE 5

Fifty parts of titanium dioxide was uniformly dispersed in a mixturecomposed of the following components:

    acrylic acid            5 parts                                               ethyl acrylate          20 parts                                              methyl acrylate         40 parts                                              2-hydroxyethyl methacrylate                                                                           15 parts                                              styrene                 20 parts                                              dodecyl mercaptan       1 part                                                benzoyl peroxide        5 parts                                           

The resulting dispersion was suspendeed in 300 parts of water maintainedat 90°C., in which 15 parts of sodium dodecylbenzene sulfonate and 100parts of calcium phosphate were dissolved and dispersed. Underagitation, 50 parts of methyl methacrylate and Emcol-CC-36 0.01 partwere added to the suspension over a period of 30 minutes, and thereaction was further conducted for 3 hours to obtain a suspension ofresin particles composed of a core polymer having a softening point of40°C. and a Tg of 150°C. and an outer polymer having a softening pointof 120°C. and a Tg of 100°C., said outer polymer covering the corepolymer.

Hydrochloric acid was added to the thus formed suspension, and it waswashed sufficiently with water and dried to obtain a flowable powderycomposition composed of spherical particles having an average particlesize of 50μ. When this composition was subjected to the storagestability test at 35°C. for 1 week, no change was observed. When thiscomposition was coated on an iron plate and baked at 180°C. for 30minutes, a smooth surface having excellent gloss and flexibility wasobtained.

EXAMPLE 6

Five parts of dicyandiamide and 80 parts of titanium dioxide weredispersed in 100 parts of a molten epoxy resin (Epikote 1001manufactured by Shell International Chemical Corp.) by means of anextruder, and the resulting composition was pulverized by means of anair jet pulverizer, to obtain a white epoxy resin powder having a sizeof 10 to 20μ.

Five parts of Ethocel 20-STD (ethyl cellulose of a softening point of135° to 145°C. manufactured by Dow Chemicals Corp.) was dissolved in 100parts of ethanol having a boiling point of 78°C. 100 parts of the abovewhite epoxy resin powder was dispersed into this solution. The resultingdispersion was dried in a spray drier in which the inlet temperature was200°C. and the outlet temperature was 75°C. Thus was obtained a whiteflowable powder having an average particle size of 20μ.

The white epoxy resin powder before the ethyl cellulose treatment andthe ethyl cellulose-treated white epoxy resin powder were stored at50°C. for one week to examine the storage stability. In the formerpowder blocking occurred, but in the latter composition of thisinvention no change was observed. When both the powdery compositionswere coated on an iron plate and heated at 150°C., in each case a goodflowability was observed and a smooth film as obtained.

EXAMPLE 7

Eighty parts of Desmophen 650 (polyester resin for urethane having asoftening point of 60°C., manufactured by Bayer AG), 20 parts ofDesmodure-AP Stable (blocked isocyanate manufactured by Bayer AG) and 2parts of dibutyl tin dilaurate (dissociation catalyst) were uniformlykneaded by means of an extruder maintained at 70°C. and the kneadedmixture was pulverized by means of an air jet pulverizer to obtain aresin powder having an average particle size of 20μ.

A styrene-maleic acid copolymer having an acid value of 110 to 115 and amelting point of 135° to 145°C. was neutralized with ammonia and a 2%aqueous solution of the neutralized copolymer was prepared. 80 Parts ofthe above resin powder was dispersed in 100 parts of the thus preparedaqueous solution, and the dispersion was dried in a spray drier in whichthe inlet temperature was 200°C. and the outlet temperature was 100°C.Thus was obtained a flowable resin powder having an average particlesize of 20μ. The resin powder before the treatment with thestyrene-maleic acid copolymer and the resin powder treated with saidcopolymer were subjected to the storage stability test at 50°C. for oneweek. In the former resin powder, caking was observed, but in the latterresin powder no change was observed, When both the resin powders werecoated on an iron plate and heated at 150°C., in each case a smooth filmwas obtained, and there was observed no difference between the two resinpowders with respect to the film-forming property.

EXAMPLE 8

Five parts of Phthalocyanine Blue was dispersed in 100 parts of adispersion of an acrylic copolymer having a softening point of 70°C. anda particle size of 10μ, which was prepared by suspension polymerizationof the following components according to a known method:water 100partsstyrene 30 partsmethyl methacrylate 20 parts2-ethylhexyl acrylate27 partsbutyl acrylate 13 partsglycidyl methacrylate 10 partsdodecylmercaptan 1 part

According to a known method, the following components were subjected tofree radical polymerization to obtain an acrylic copolymer having asoftening point of 120°C. and an acid value of 65:

    styrene                40 parts                                               methyl methacrylate    27 parts                                               butyl acrylate         25 parts                                               acrylic acid           8 parts                                                isopropyl alcohol      100 parts                                          

10 Parts of the thus formed acrylic copolymer was neutralized withtriethylamine and a 10% aqueous solution of the neutralized copolymerwas prepared. 50 Parts of the solution was mixed with the abovedispersion and the resulting liquid mixture was spray dried in a spraydrier in which the inlet temperature was 200°C. and the outlettemperature was 90°C. to obtain a blue, flowable resin powder composedof a core polymer having a softening point of 70°C. and an outercovering polymer having a softening point of 120°C. When this resinpowder was heated at 50°C. for 1 week, occurrence of such undesiredphenomena as caking was not observed. When this resin powder was coatedon an iron plate and baked at 180°C, for 20 minutes, a smooth film wasobtained.

EXAMPLE 9

A copolymer having a softening point of 50°C. was prepared from thefollowing components according to a known bulk polymerization method:

    styrene                 40 parts                                              methyl methacrylate     20 parts                                              2-ethylhexyl acrylate   30 parts                                              butyl acrylate          10 parts                                          

100 Parts of the thus prepared copolymer, 30 parts of cellulose acetatebutyrate (1/2 second) and 100 parts of titanium dioxide were meltkneaded by means of a heating roll, and the kneaded mixture waspulverized by an air jet pulverizer so that the average particle size ofthe pulverized product was 20μ. 100 Parts of the resulting white powderwas dispersed in 100 parts of a 5% solution of ethyl cellulose inethanol, and the dispersion was spray dried to obtain a white, flowablepowder, the surface of which was covered with ethyl cellulose. When thepowder not treated with ethyl cellulose was heated at 50°C. for oneweek, occurrence of caking was observed, but in the case of the powdertreated with ethyl cellulose no change was observed. When both thepowders were coated on a substrate and heated at 150°C. for 30 minutes,in each case a white film having excellent smoothness and gloss wasobtained.

EXAMPLE 10

One hundred parts of Pentacite P-406 (natural resin-modifiedpentaerythritol resin having a softening point of 80 to 90°C,manufactured by Dainippon Ink & Chemical, Inc.) was melted in watermaintained at a temperature higher than 95°C., and agitation wasconducted so that the dispersed particles had a size of 20 to 30μ. Whencooling was conducted, there was obtained a suspension of fineparticles. 1 Part of Cellosize WP (hydroxyethyl cellulose of a softeningpoint of 140°C. manufactured by Union Carbide Corporation) was dissolvedin the suspension, and it was spray dried to obtain a powder, thesurface of which was coated with hydroxyethyl cellulose. No cakingoccurred in this powder during storage, and when this powder was heatedat 140°C. for 30 minutes, a smooth film was obtained.

EXAMPLE 11

One hundred parts of titanium dioxide was dispersed in 100 parts of anemulsion of an acrylic copolymer having a softening point of 65°C. whichwas obtained by emulsion polymerization the following components:laurylmethacrylate 30 partsstyrene 20 partsethyl acrylate 30parts2-hydroxyethyl methacrylate 20 partsdodecyl mercaptan 2 partswater100 parts

Then, 2 parts of the same hydroxyethyl cellulose as used in Example 10was dissolved in the above dispersion, and it was spray dried to obtaina white, flowable powder having an average particle size of 40μ. In thispowder no caking was observed during storage, and this powder gave agood film when heated.

EXAMPLE 12

Twenty parts of Hitanol 2180 (phenol resin having a softening point of65° to 80°C. manufactured by Hitachi Chemical Co., Ltd.) was dissolvedin 100 parts of a 50% xylene solution of the same epoxy resin as used inExample 6, and 60 parts of titanium dioxide was kneaded and dispersed inthe above solution according to a known method. The resulting dispersionwas emulsified in a dispersion medium composed of 100 parts of water, 10parts of acetone and 8 parts of Epogen S-5 (non-ionic surfactantmanufactured by Dai-Ichi General Co., Ltd.), and the organic solventcomponent was removed by heating under reduced pressure with stirring.There was obtained a white epoxy-phenol resin suspension. To thissuspension was added 50 parts of a 10% aqueous solution of the samewater-soluble acrylic copolymer used in Example 8. The suspension wasspray dried to obtain a white, flowable powder composed of a white corecomponent of the epoxy-phenol resin having a softening point of 70°C.and an outer covering component of the acrylic resin having a softeningpoint of 120°C. When this powder was heated at 50°C. for 1 week,occurrence of such undesired phenomena as caking was not observed. Whenthis powder was coated on a substrate and baked at 180°C. for 20minutes, a white film having excellent gloss and smoothness wasobtained.

EXAMPLE 13

Two parts of carbon black was kneaded and dispersed in 50 parts of SuperBekamin 820 (60% solution of a melamine resin having a softening pointof 10°C. in a 1:1 mixed solvent of xylol and butanol; manufactured byDai Nippon Ink & Chemical, Inc.), and the mixture was emulsified anddispersed in water. When the solvent (xylol and butanol) was removed byheating in vacuo, an aqueous dispersion of a black powder of themelamine resin was obtained.

Separately, a mixture of 450 parts of coconut oil, 100 parts oftrimethylol propane, 201 parts of pentaerythritol, 525 parts of phthalicacid, 144 parts of trimellitic anhydride and 600 parts of ethylcellosolve was condensed according to a conventional method to obtain analkyd resin solution having an acid value of 60 and a non-volatilecontent of 70% (the softening point of the resin is 120°C.). 3 parts ofcarbon black was kneaded and dispersed in 100 parts of the thus obtainedresin solution, and the mixture was neutralized to a pH of 8 with use ofammonia.

The thus neutralized resin solution was dissolved in the above aqueousdispersion, and the resulting liquid mixture was spray dried to obtain ablack, flowable powder. No caking occurred when it was stored, and asmooth film having solvent resistance was obtained by coating it on asubstrate and heating it at 150°C. for 30 minutes.

EXAMPLE 14

A mixture composed of 30 parts of styrene, 20 parts of methylmethacrylate, 5 parts of 2-hydroxyethyl methacrylate 15 parts ofglycidyl methacrylate, 20 parts of ethyl acrylate, 10 parts of butylacrylate and 1 part of dodecyl mercaptan was suspension polymerizedaccording to a known method, and the resulting reaction mixture wasdried to obtain a powder of an acrylic copolymer having a softeningpoint of 65°C. and a particle size of 500μ.

A mixture composed of 60 parts of the above acrylic copolymer, 15 partsof sebacic acid, 25 parts of titanium dioxide and 1 part of a flowregulator was melted and kneaded by means of an extruder, and thekneaded mixture was pulverized by means of a turbo-mill to obtain awhite fine powder having an average particle size of 50μ.

100 parts of the thus obtained powder was dispersed in 150 parts of a 1%solution of ethyl cellulose in ethanol, and the dispersion was spraydried to obtain a white powdery composition having flowability, thesurface of which was covered with ethyl cellulose. This composition hadexcellent storage stability. When this composition was coated on an ironplate and baked at 180°C. for 20 minutes, a smooth film having excellentfinish characteristics was obtained.

What is claimed is:
 1. A powdery coating composition comprising polymerparticles having an average particle size within the range of 0.5 to200μ and composed of, as a core component, an acrylic copolymer having asoftening point not exceeding 90°C. and containing 5 to 60% by weight ofunits of a monomer whose homopolymer has a Tg lower than 0°C. and, as anouter component, an acrylic polymer having a softening point of at least100°C. and containing at least 60% by weight of units of a monomer whosehomopolymer has a Tg higher than 50°C., wherein the weight ratio of thecore component to the outer component is within a range from 50/50 to100/1, and said outer component covers said core component.
 2. Thecomposition of claim 1 wherein the core component comprises a copolymerof 2-ethylhexyl methacrylate, methyl methacrylate, styrene,2-hydroxyethyl methacrylate and methacrylic acid and the outer componentcomprises a copolymer of 2-ethylhexyl methacrylate, N-methylolacrylamide, styrene, methyl methacrylate, butyl methacrylate andmethacrylic acid.
 3. The composition of claim 1 wherein the corecomponent comprises a copolymer of ethyl acrylate, lauryl methacrylate,β-hydroxyethyl methacrylate, styrene and butyl methacrylate and theouter component comprises a copolymer of methyl methacrylate, styreneand trimethylamine methacrylamide.
 4. The composition of claim 1 whereinthe core component comprises a copolymer of lauryl methacrylate, n-butylmethacrylate and styrene and the outer component comprises a copolymerof methyl methacrylate, styrene and butyl acrylate.
 5. The compositionof claim 1 wherein the core component comprises a copolymer of methylmethacrylate and lauryl methacrylate and the outer component comprises acopolymer of methyl methacrylate and styrene.
 6. The composition ofclaim 1 wherein the core component comprises a copolymer of acrylicacid, ethyl acrylate, methyl acrylate, 2-hydroxyethyl methacrylate andstyrene and the outer component comprises poly (methyl methacrylate). 7.The composition of claim 1 wherein the core component comprises acopolymer of styrene, methyl methacrylate, 2-ethylhexyl acrylate, butylacrylate and glycidyl methacrylate and the outer component comprises acopolymer of styrene, methyl methacrylate, butyl acrylate and acrylicacid.